Indicate the steps in the preparation of:

(i) K₂Cr₂O₇ from chromite ore. (ii) KMnO4 from pyrolusite ore.

(i) the chromite ore (FeCr₂O₄) is fused with sodium hydroxide (NaOH)

4FeCr₂O₄ +16 NaOH+ 7O₂ → 8Na₂CrO₄ +2Fe₂O₃ +8H₂O

The yellow solution of sodium chromate is then filtered and acidified with sulphuric acid giving its dichromate.

2Na₂CrO₄ +H₂SO₄→ Na₂Cr₂O₇ + Na₂SO₄ +H₂O

On cooling, sodium sulphate crystallizes out as Na₂SO₄.10H₂O and is removed.

Na₂Cr₂O₇ +2KCl → K₂Cr₂O₇ + 2NaCl

(ii) the pyrolusite ore(MnO₂) is oxidised in the presence of potassium hydroxide by heating.

2MnO₂ + 4KOH +O₂→ 2K₂MnO₄ + 2H₂O

The green potassium manganate (K₂MnO₄) is then treated with a current of chlorine or ozone to oxidise potassium manganate to potassium permanganate.

2K₂MnO₄ +Cl₂→ 2KCl +2KMnO₄

K₂MnO₄ +O₃ +H₂O → 2KMnO₄ +2KOH +O₂